Tin-containing iron base powder and process for making

ABSTRACT

The invention provides a tin-containing iron base powder comprising iron base particulates each having tin-rich portions on the surface in which at least a part of the tin forms a compound with iron, wherein the total tin content ranges from 1 to 20% by weight of the powder. The invention also provides a process for making such a tin-containing iron base powder by mixing an iron base powder with a powder tin compound capable of generating metallic tin upon thermal decomposition, and heating the mixture to a temperature of 450° to 700° C. in a reducing or non-oxidizing atmosphere. Using this tin-containing iron base powder as a sintering material, there can be obtained sintered products having a high density and excellent magnetic properties.

This application is a continuation of now abandoned application Ser. No.908,957 filed Sept. 18, 1986 which in turn is a continuation of Ser. No.700,483, filed Feb. 1, 1985 now abandoned.

TECHNICAL FIELD

This invention relates to powders which are sintering stocks forsintered iron base materials for use as mechanical parts, soft magneticparts or the like, and a process for making the powders.

BACKGROUND ART.

Electrical sheets, silicon steel sheets and the like have heretoforebeen widely used as soft magnetic parts such as cores in electricapparatus such as electric motors as is well known in the art. Recently,sintered magnetic materials formed by compacting and sintering ironpowder have progressively replaced the electrical sheets and siliconsteel sheets. These sintered materials have some advantagescharacteristic of powder metallurgy including an increased percent yieldbased on the stock, a low processing cost, and an increased degree offreedom in shape, but have the disadvantage that their magneticcharacteristics are imperatively inferior to those of electrical sheetsand silicon steel sheets due to residual voids in the sinteredmaterials.

To overcome the drawbacks of sintered iron base materials as mentionedabove, attempts have been made to add a variety of additives. Among suchadditives, tin (Sn) forms a liquid phase at a relatively lowtemperature. If tin is added, a liquid phase is created during sinteringand tin forms a solid solution with iron to allow α-phase iron todevelop during sintering, resulting in an increased sinter density,reduced influence of voids, and promoted growth of α-phase crystals, andhence, the possibility of achieving excellent magnetic characteristics.If high density sinters are made by adding tin, then it is expectable toapply them to sintered mechanical parts requiring wear resistance andhigh strength.

Known among processes for adding tin to sintered iron-base material is aprocess comprising mixing a tin powder with an iron powder, compactingthe mixture and sintering it, as disclosed in Japanese PatentApplication Kokai No. 48-102008. In this process, however, since tin ismelted during sintering to penetrate between iron particulates to spreadthe interstices between them and depleted voids are left where tinparticulates have occupied before melting, the sinter density is notsufficiently increased in practice, failing to provide satisfactorymagnetic characteristics.

In order to overcome such problems, it may be contemplated to use apowder iron alloy which has previously been alloyed. However, alloyingwith tin makes iron base powder harder to considerably deteriorate itscompressibility to provide a reduced compact density although thedevelopment of tin-depleted voids is prevented. It is thus eventuallydifficult to provide a high sinter density.

It may also be contemplated that if very finely divided tin is used asthe metallic tin powder to be added to and blended with an iron powder,then depleted voids of a substantial size are not left even aftermelting of tin during sintering so that uniform sintering may take placeto yield a coherent sinter. However( atomizing and trituratingtechniques normally employed in the preparation of tin powder aredifficult to effectively produce such very finely divided tin.

It is, therefore, a primary object of the present invention to providean Sn-containing iron base sintering powder stock which can be convertedinto a coherent sinter having improved magnetic characteristics.

It is a secondary object to provide a process for efficiently making anSn-containing iron base sintering powder stock in an industrial scale,the powder stock being convertible into a coherent sinter havingimproved magnetic characteristics.

It is a further object of the present invention to provide anSn-containing iron base sintering powder stock which may be convertedinto Sn-containing iron base sinters exhibiting high strength and highwear resistance in their applications other than as megnetic parts, forexample, application as mechanical parts or the like, as well as aprocess for making such a powder stock.

DISCLOSURE OF THE INVENTION

Making extensive experiments and investigations in order to attain theabove-mentioned objects, the inventors have found that in preparing asintering iron-tin base material, coherent sinters having improvedmagnetic characteristics can be produced using as a tin-providingsintering powder stock, a composite powder comprising iron particulateseach having tin-rich portions formed on the surface in which at least apart of the tin forms a compound with iron. It has also been found thatthe above-mentioned composite powder is easily prepared by mixing groundiron with a powder tin compound which is thermally decomposable into Sn,for example, tin oxides, and reducing the resultant mixture. It shouldbe noted that the desired effect is achievable by controlling thecontent of tin in the composite powder to 1 to 20% by weight.

Accordingly, the tin-containing powder according to the presentinvention is a tin-containing iron base powder having improved sinteringproperty, characterized by comprising iron base particulates each havingtin-rich portions at the surface in which at least a part of the tinforms a compound with iron, wherein the total content of tin is in therange of 1 to 20% by weight of the powder.

The powder making process of the present invention is a process formaking the above-described tin-containing iron base powder,characterized by mixing an iron base powder with at least one powderselected from the group consisting of tin oxide, tin chloride, tinoxalate, tin nitrate, tin sulfate, and tin sulfide powders in an amountof 1 to 20% by weight calculated as tin, and effecting heat treatment ata temperature of 450° to 700° C. in a non-oxidizing or reducingatmosphere.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is the phase diagram of Fe-Sn system;

FIG. 2 is a scanning electron photomicrograph of Fe-Sn compositeparticulate surface;

FIG. 3 is a schematic view showing a portion of Fe-Sn compositeparticulate surface;

FIGS. 4A to 4C are photographs by an X-ray microanalyzer of Fe-Sncomposite particulate surface, FIG. 4A being a secondary electron image,FIG. 4B being an X-ray image of Sn character, and FIG. 4C being an X-rayimage of Fe character;

FIG. 5 is the phase diagram of Fe-P system;

FIGS. 6A and 6B are diagrams showing the content of tin in compositepowders prepared in Example 1 in relation to the magnetic flux densityB₂₅ and iron loss W_(10/50) in the resulting sinters, respectively;

FIGS. 7A to 7C are diagrams showing the reducing heat treatingtemperatures used in Example 2 in relation to the oxygen content ofcomposite powders, compact density and sinter density, respectively; and

FIGS. 8A to 8C are diagrams showing the heat treating temperatures usedin Example 5 in relation to the oxygen content of composite powders,compact density and sinter density, respectively.

BEST MODE FOR CARRYING OUT THE INVENTION

According to the present invention, there is provided as a sinteringpowder stock for producing tin-containing iron base sinters, a compositeiron base powder comprising iron base particulates each having tin-richportions at the surface in which at least a part of the tin forms acompound with iron, wherein the total content of tin is in the range of1 to 20% by weight of the powder. This composite powder is subject tosintering alone or in admixture with iron powder and/orphosphorus-containing powder (for example, iron-phosphorus alloy powder)as will be described later. When the composite powder is used insintering, tin which is finely distributed, rapidly diffuses into theinterior of powder particulates (composite powder particulatesthemselves and iron powder particulates mixed therewith) even if tin ismelted during sintering. Not only the behavior of tin to spread theinterstices between particulates and the behavior or leaving large voidsare precluded, but also alloying occurs fast and uniformly to facilitatethe development of α-phase to promote sintering so that sinters having ahigh density and hence, improved mechanical and magnetic properties maybe obtained. Moreover, the addition of tin allows crystal particles togrow larger, resulting in further improved magnetic properties.

With tin contents of the composite powder of less than 1% by weight, theFe-Sn phase diagram shown in FIG. 1. suggests that even when thecomposite powder alone is sintered, the development of α-phase does notoccur at usual sintering temperatures in the range of 950° to 1300° C.and thus the promotion of sintering is not fully accomplished. On theother hand, it is difficult to incorporate more than 20% by weight oftin into iron base powder particulates as tin-rich portions at thesurface. In this case, tin agglomerates during sintering to give rise tobehavior as occurring when a coarse tin powder is admixed, failing toeffectively increase the density of sinters.

The above-mentioned composite powder may desirably be prepared by mixinga powder having a major proportion of iron (to be referred to asiron-base matrix powder, hereinafter) with a tin compound powder havinga particle size equal to or smaller than that of the iron-base matrixpowder particulates, and heating the mixture at a temperature range offrom 450° to 700° C. in a reducing or non-oxidizing atmosphere todecompose the tin compound. The tin compounds used herein may be anydesired one as long as it is decomposed by heating to generate tin, andspecial mention may be made of one or mixtures of more than one selectedfrom tin oxide (SnO or SnO₂), tin hydroxide (Sn(OH)₂ or SnO₂ ·nH₂ o),tin chloride (SnCl₂ or SnCl₄ with or without water of crystallization),tin oxalate (C₂ O₄ Sn) tin nitrate (Sn(NO₃)₂ or Sn(NO₃)₄ with or withoutwater of crystallization), tin sulfate (SnSO₄), and tin sulfide (SnS orSnS₂). All these tin compounds are markedly more brittle than metallictin so that they may be readily comminuted. Such a comminuted tincompound may be mixed with an iron-base matrix powder and heated in areducing or non-oxidizing atmosphere to produce a composite powderconsisting essentially of iron-base matrix powder particulates havingtin-rich portions uniformly formed or distributed at the surfacethereof.

The iron-base matrix powder used herein is basically a powder having amajor proportion of iron and desirably, substantially free of Sn.Examples of the iron-base matrix powder include atomized pure ironpowders having an iron content of at least 99.0% and containing asimpurity elements not more than 0.02% of carbon (C), not more than 0.10%of silicon (Si), not more than 0.15% or not more than 0.35% of manganese(Mn), not more than 0.020% of phosphorus (P), and not more than 0.020%of sulfur (S); reduced iron powders having an iron content of at least98.5% by weight and containing as impurities not more than 0.05% ofcarbon, not more than 0.15% of silicon, not more than 0.40% ofmanganese, not more than 0.020% of phosphorus, and not more than 0.020%of sulfur; low alloy steel powders containing as an alloying constituentat least one selected from 1.3 to 1.6% of nickel (Ni), 0.2 to 0.6% ofmolybdenum (Mo), 0.4 to 0.7% of copper (Cu), and 0.9 to 1.2% of chromium(Cr) and the balance being substantially iron and concomitantimpurities; and the like.

At heat treating temperatures of lower than 450° C., as will be latershown in Example 2, reduction does not fully proceed to leave the hardtin compound which would cause molds to be worn during compacting andprevent the compact density from being fully increased, resulting insinters of a relatively low density. On the other hand, at heat treatingtemperatures of higher than 700° C., as will also be later shown inExample 2, tin is extremely diffused and alloyed into the iron-basematrix powder to make the powder harder, resulting in a reduction incompact density, and hence, sinter density. For these reasons, the heattreating temperature is limited to the range of from 450° to 700° C.

Now, it will be further described in detail how the tin-containingcomposite iron base powder is produced by the process of the presentinvention.

At the outset, mixing an iron-base matrix powder with a tin compoundresults in a mixture in which the iron value is in macroscopic admixturewith the tin value. In this case, the finer the tin compound powder, themore intimate the mixture becomes. However, because of furthercomminutation in the subsequent steps, relatively coase tin compoundpowder may be used as long as its particle size is equal to or smallerthan that of the iron-base matrix powder. The thus obtained mixture isheated to a temperature of at least 450° C. in reducing or non-oxidizingatmosphere to decompose the tin compound to yield metallic tin. Sincethe heating temperature is enough higher than the melting point of tin,the tin generated is instantaneously converted into molten tin whichexhibits good wetting properties to the iron-base matrix powder andpartially or entirely covers the surface of matrix powder particulates.

A part of the molten tin reacts with iron to form an iron-tin compoundin solid state at the surface of iron-base matrix powder particulates,forming Sn-rich portions on the powder particulate surface.

FIG. 2 is a scanning electron photomicrograph of the surface of iron-tincomposite powder particulates produced in this way, and FIG. 3 is aschematic view showing a portion of the particulate surface. In FIG. 3,reference numeral 1 designates ridges and recesses on the particulatesurface, and minute precipitates 2 in the form of very fine white spotson the particulate surface consist essentially of iron-tin compounds.This is attested by the photographs by an X-ray microanalyzer and X-rayidentification analysis of iron-tin composite powder particulates asshown in FIGS. 4A to 4C. More specifically, FIG. 4A is a secondaryelectron image, FIG. 4B is the corresponding X-ray image of Sncharacter, and FIG. 4C is the corresponding X-ray image of Fe character.The X-ray analysis shows that the fine product on the particulatesurface consists predominantly of iron-tin compounds (FeSn or Fe₃ Sn₂,or FeSn₂ or the like) and metallic tin is locally identified.

In the above-described process, the lower the heating temperature andthe shorter the heating time, the reaction of tin with iron terminatesin more incomplete state and depending on the extent of the reaction,metallic tin may sometimes remain on the surface of iron-base matrixpowder particulates. Alternatively, the residual tin which is leftwithout covering the surface of iron base matrix powder particulates (oriron-tin compounds resulting from the reaction of this residual tin withiron) may sometimes take the form of grains attached to the surface ofiron-base matrix powder particulates. For improved powder quality, it ispreferred to form or distribute tin-rich portions on the iron-basematrix powder particulate surface as uniformly as possible and for thesegregated tin to be of iron-tin compounds. However, if the heatingtemperature is increased above 700° C. merely for these purposes, thenthe tin segregated on the iron-base matrix powder particulate surface isreadily diffused into the particulate interior and alloyed, resulting inhardened powder particulates. It is thus rather unavoidable thatmetallic tin partially remains.

Depending on the particular element contained in the iron-base matrixpowder, the tin rich portions can contain a third element other thaniron and tin.

The iron-tin composite powder proposed by the present invention issubstantially improved in sinterability for the following two reasons.First, the fine distribution of tin on the surface of iron-base matrixpowder particulates, even if the tin should be metallic tin, preventslarge tin-depleted voids from being left during sintering, resulting inmore coherent sinters. Secondly, at least a part of the tin value ispresent in the form of an iron-tin compound having a higher meltingpoint so that the diffusion of tin into iron proceeds to some extentuntil the development of a liquid phase during sintering, precluding thepnenomenon that large tin-depleted voids are left as a result of instantmelting of tin-rich portions.

The following discussion is made about the difference of the process ofthe present invention from other well-known processes for formingtin-rich portions on iron-base matrix powder particulate.

First, Japanese Patent Publication No. 43-14571 discloses a process forimproving the moldability of stainless steel powder by immersing thestainless steel powder in a tin plating bath to effect tin platingtreatment on the powder surface. In this case, however, tin is presentof the steel powder surface in the form of metallic tin, and animprovement in sintering properties due to the containment of tin in theform of an iron-tin compound as described earlier is not expectable.

Also, Japanese Patent Application Kokai No. 54-19458 discloses a processfor improving the moldability of an alloyed steel powder by mixing thesteel powder with metallic tin and heat treating the mixture. However,this process has the following problems as compared with the process ofthe present invention using a tin compound.

First, the softness of metallic tin leads to the difficulty of finelydividing it by grinding. If ground tin has a particle size larger thanthat of the iron-base matrix powder, more iron-base matrix powderparticulates have not tin-rich portions on the surface. Secondly, when amixture of metallic tin with an iron-base matrix powder is heated, tinreacts with iron to form a compound. Great difficulty is thus imposed onthe choice of a proper heating temperature condition to permit tin toremain on the surface of iron-base matrix powder particulates. Theinventor has found through experimentation that in the heat treatment ofa mixture of an iron-base matrix powder with a more finely dividedmetallic tin powder, heating temperatures of 230° to 450° C. allow thetin to melt and cover a part of the surface of iron-base matrix powderparticulates, but do not allow the tin to form compound with iron. Animprovement in sintering properties due to the formation of an iron-tincompound as described earlier is not expectable. At heating temperaturesin excess of 450° C., the diffusion of tin into iron-base matrix powderparticulates commences and the solid solution of tin into theparticulates makes them harder to deteriorate compressibility. Incontrast, when a mixture of an iroh-base matrix powder with a tincompound is heated as in the present invention, the tin is enriched asan iron-tin compound on the surface of iron-base matrix powderparticulates in the temperature range of from 450° to 700° C. becausethe tin compound admixed does not melt at temperatures above 230° C.,the melting point of tin, preventing tin is solid phase from diffusinginto the iron-base matrix powder particulates.

Furthermore, iron-copper composite powder preparing techniques similarto the iron-tin composite powder making process of the present inventionare disclosed in Japanese Patent Application Kokai Nos. 53-92306 and56-38401. These techniques have a fundamental difference from theprocess of the present invention as described below. First, the priorart processes for making iron-copper composite powders use a heatingtemperature lower than the melting point of copper for the purpose ofintegration whereas the process of the present invention uses a heatingtemperature higher than the melting point of tin, which enables moreuniform distribution of tin as the iron-base matrix powder is oncecovered with tin in the process. Secondly, copper is used in the form ofmetallic copper in the prior art processes for making iron-coppercomposite powders whereas at least a part of the tin is present in theform of iron-tin compounds in the process of the present invention.Consequently, the resulting iron-copper composite powders do not gain asubstantial improvement in sintering properties and specifically, sinterdensity over conventional powder mixing techniques whereas a substantialimprovement is achieved by the process of the present invention.

It should be understood that one of the important applications of theiron-tin composite powder produced by the process of the presentinvention includes sintered magnetic parts as described earlier. In thiscase, more excellent properties are obtainable by producing sinteredbodies while simultaneously adding P which is known to improve magneticproperties.

In the practice of the process of the present invention, the compositepowder as defined above may be compacted and sintered alone or inadmixture with and iron powder. When it is desired to produce sinteredmaterials of iron-tin-phosphorus system, the composite powder as definedabove may be mixed with a phorphorus-containing powder as a phorphorussource, for example, iron-phosphorus alloy powder, red phosphorus powderor the like, or the composite powder may be mixed with aphosphorus-containing powder and an iron powder before the resultantmixture is compacted and sintered. It is, or course, included with thescope of the present invention to incorporate a predetermined amount ofa lubricant before compacting.

The ultimately obtained sinters may desirably have a tin content in therange of 1 to 10% by weight. Tin contents of sinters of less than 1% byweight presuppose tin contents of the starting composite powder of lessthan 1% by weight, which is too low to achieve promoted sintering asdescribed above. If the tin content of sinters exceeds 10% by weight, asseen from the Fe-Sn phase diagram shown in FIG. 1, a non-magneticintermetallic compound phase (FeSn) precipitates upon colling aftersintering, resulting in sinters with poor magnetic properties.

In the case of iron-tin-phosphorus system sinters, the phosphoruscontent of sinters is desirably in the range of from 0.1 to 2% byweight. If the phosphoru content is less than 0.1% by weight, asunderstood from the Fe-P system phase diagram shown in FIG. 5, α-phasedoes not develop at usual sintering temperatures of 950° to 1300° C.,failing to obtain the effect of promoted sintering due to the additionof phosphorus. Of course, the co-existence of tin changes the quantityof phosphorus required to develop α-phase. With sufficiently smallquantities of phosphorus to prevent the development of α-phase iniron-phosphorus system, it is estimated that the additive effect is alsovery slight for an iron-tin-phosphorus system. On the other hand, theaddition of a powder to be a phosphoru source detracts from thecompressibility of a mixed powder as is well known and extremely reducescompact density particularly at phosphorus contents in excess of 2% byweight, resulting in sinters with reduced sinter density and increaseddimensional change before and after sintering, which leads todeteriorated dimensional accuracy of sinter.

Examples of the present invention are presented below together withcomparative examples.

EXAMPLE 1

An atomized iron having a particle size of -80 mesh was mixed with anSnO powder having particle size of -325 mesh in varying proportions, andheated at 600° C. for 60 minutes in a stream of decomposed ammonia gas,obtaining iron-tin composite powders having varying tin contents. Eachof the powders was mixed with 1% by weight of zinc stearate, compactedunder a compression pressure of 7 t/cm², and then sintered at 1300° C.for 60 minutes in a stream of decomposed ammonia gas. In FIGS. 6A and6B, the magnetic flux density B₂₅ (magnetic flux density in a magneticfield of 25 Oe) and iron loss W_(10/05) (iron loss at a magnetic fluxdensity of 10 kG and a frequency of 50 Hz) of the resulting sinteredproducts are plotte as a function of tin contents of the compositepowder.

As evident from FIG. 6, at Sn contents in excess of 1% by weight, thetendency that magnetic flux density increases and iron loss decreasesbecomes outstanding. It is to be noted that magnetic flux density turnsdown when the Sn content exceeds 5% by weight and diminishes to a levellower than in the absence of Sn when the Sn content exceeds 20% byweight. On the other hand, iron loss undesirably increases when the Sncontent exceeds 20% by weight probably because Sn in excess of 20% byweight cannot be uniformly distributed over the surface of iron powderparticulates and large voids are thus left in the resulting sinteredporducts.

EXAMPLE 2

An atomized iron having a particle size of -80 mesh was mixed with anSnO₂ or SnO powder having a particle size of -325 mesh in an amount of4% by weight calculated as tin, and subjected to reducing treatment byheating to different temperatures within the range of 400° to 800° C.for 60 minutes in a stream of decomposed ammonia gas, obtaining iron-tincomposite powders. In FIGS. 7A to 7C, the oxygen content in compositepowders, the density of compacts after compression and before sintering,and the density of ultimately sintered products are plotted as afunction of reducing temperatures used in the reducing treatment of thepowder mixture. For comparison sake, FIGS. 7B and 7C also show thecompact density and sinter density obtained by mixing the same atomizediron with 4% by weight of ground metallic tin of -250 mesh andcompacting and sintering in the same manner as above according to theprior art process (process described in Japanese Patent ApplicationKokai No. 48-10028). It is to be noted that compacting was carried outin the presence of 1% of zinc stearate and under a compression pressureof 7 t/cm² and sintering was carried out at 1150° C. for 60 minutes indecomposed ammonia gas.

As evident from FIG. 7, by subjecting the powder mixtures of tin oxideswith iron powder to reducing treatment at temperatures of 450° to 700°C., the tin oxides were fully reduced to produce composite powdershaving tin-rich portions formed on the surface of iron powderparticulates. Consequently, there were obtained sintered products havinga significantly higher density than achieved in the prior art process.

EXAMPLE 3

An atomized iron having a particle size of -80 mesh was mixed with anSnO powder having a particle size of -325 mesh in a amount of 4% byweight calculated as tin, and heated at 600° C. for one hour in a streamof decomposed ammonia gas, preparing an iron-tin composite powder. Theresulting powder to which 1% by weight of zinc stearate was added as alubricant was compacted under a compression pressure of 7 t/cm². Thecompact was then sintered at 1200° C. for one hour in a stream ofdecomposed ammonia gas to yield a sintered iron-tin product. The sinterhad a ring shape having an outer diameter of 38 mm, an inner diameter of25 mm, and a height of 6.5 mm. The density of the sinter was measuredand it was also determined for magnetic properties including magneticflux density B₂₅, coercive force Hc, maximum magnetic permeabilityμ_(max), and iron loss W_(10/50). The results are shown in Table 1.

Comparative Example 1

According to the process described in Japanese Patent Application KokaiNo. 48-10028, an atomized iron having a particle size of -80 mesh wasmixed with 4% by weight of a tin powder of -250 mesh, 1% by weight ofzinc stearate was added as an additive, and the resulting powder wascompacted and sintered in the same manner as in Example 2 to produce aniron-tin sinter. The density and various magnetic properties of thesinter are also shown in Table 1.

EXAMPLE 4

To the same iron-tin composite powder as prepared in Example 3 was addedan iron-phosphorus alloy powder of -325 mesh (phosphorus content 16% byweight) as a phosphorus source in such an amount as to give a phosphoruscontent of 0.6% by weight based on the powder mixture. Further, 1% byweight of zinc stearate was added thereto as a lubricant. The mixturewas then compacted and sintered in the same manner as in Example 2 toproduce an iron-tin-phosphorus sinter. The density and various magneticproperties of the sinter are also shown in Table 1.

Comparative Example 2

An atomized iron having a particle size of -80 mesh; was mixed with atin powder of -250 mesh and an iron-phosphorus alloy powder of -325 mesh(phosphorus content 16% by weight) in such amounts as to give a tincontent of 4% by weight and a phosphorus content of 0.6% by weight basedon the powder mixture. Further 1% by weight of zinc stearate was addedthereto as a lubricant. The mixture was then compacted and sintered inthe same manner as in Example 3 to produce an iron-tin-phosphorussinter. The density and various magnetic properties of the sinter arealso shown in Table 1.

                  TABLE 1                                                         ______________________________________                                                       Magnetic properties                                                   Sinter density                                                                          B.sub.25                                                                             Hc            W.sub.10/50                                    (g/cm.sup.3)                                                                            (kG)   (Oe)    μmax                                                                             (W/kg)                                  ______________________________________                                        Example 3                                                                              7.49        14.5   0.88  7560  36.4                                  Comparative                                                                            7.35        13.9   1.16  5890  37.2                                  Example 1                                                                     Example 4                                                                              7.46        14.4   0.85  8790  29.6                                  Comparative                                                                            7.22        13.4   1.12  5090  30.2                                  Example 2                                                                     ______________________________________                                    

As seen from Table 1, the sintered products obtained in Examples 3 and 4according to the present invention have a higher sinter density than thesintered products of the same compositions obtained in ComparativeExamples 1 and 2 according to the prior art process, and hence exhibitimproved magnetic properties including high magnetic flux density, lowcoercive force, high permeability, and low iron loss.

EXAMPLE 5

An atomized iron having a particle size of -80 mesh was mixed with an H₂SnO₃ (metastannic acid, one of tin hydroxides) having a particle size of-325 mesh in an amount of 4% by weight calculated as tin, and then heattreated at different temperatures within the range of from 400° to 800°C. for 60 minutes in a stream of decomposed ammonia gas, obtainingiron-tin composite powders. In FIGS. 8A to 8C, the oxygen content in thecomposite powders, the density of compacts after compression and beforesintering, and the density of sinters at the end of sintering areplotted as a function of temperatures used in the reducing treatment ofthe powder mixture. For comparison sake, FIGS. 7B and 7C also show thecompact density and sinter density obtained by mixing the same atomizediron with 4% by weight of ground metallic tin of -250 mesh andcompacting and sintering in the same manner as above according to theprior art process (process described in Japanese Patent ApplicationKokai No. 48-10028). It is to be noted that compacting was carried outin the presence of 1% of zinc stearate and under a compression pressureof 7 t/cm² and sintering was carried out at 1150° C. for 60 minutes indecomposed ammonia gas. As evident from FIG. 8, by subjecting the powdermixture of metastannic acid with iron powder to reducing treatment attemperatures of 450° to 700° C., the metastannic acid is fully reducedto produce composite powders having tin-rich portions developed on thesurface of iron powder particulates. Consequently, there were obtainedsintered products having a significantly higher density than achieved inthe prior art process.

EXAMPLE 6

An atomized iron having a particle size of -80 mesh as the iron-basematrix powder was mixed with predetermined amounts of tin-containingpowders (all having a particle size of -200 mesh) and treated under theconditions shown in Table 2, obtaining iron-tin composite powderscontaining 4% by weight of tin. Among them, powders A, B, C, D, and Eare in accord with the present invention and powders F and G arecomparative examples.

                  TABLE 2                                                         ______________________________________                                                Tin-containing                                                                              Heating   Heating                                       Symbol  powder        atmosphere                                                                              temperature (°C.)                      ______________________________________                                        A       SnCl.sub.2 2H.sub.2 O                                                                       N.sub.2   600                                           B       SnC.sub.2 O.sub.4                                                                           H.sub.2   600                                           C       SnS.sub.2     H.sub.2   600                                           D       Sn(NO.sub.3).sub.4                                                                          H.sub.2   600                                           E       50 wt % SnSO.sub.4 +                                                                        vacuum    600                                                   50 wt % SnCl.sub.4                                                                          (10.sup.-3 Torr)                                        F       Sn            N.sub.2   400                                           G       Sn            N.sub.2   600                                           ______________________________________                                    

The resulting powders compacted under a compression pressure of 7 t/cm²after 1% by weight of zinc stearate was added thereto as a lubricant.Thereafter, the compacts were sintered at 1200° C. for one hour in astream of decomposed ammonia gas, obtaining sintered iron-tin products.The sinters had a ring shape having an outer diameter of 38 mm, an innerdiameter of 25 mm, and a height of 6.5 mm. The density of the sinterswas measured and they were determined for magnetic properties, that is,magnetic flux density B₂₅ and coercive force H_(c), with the resultsshown in Table 3.

                  TABLE 3                                                         ______________________________________                                                Compact density                                                                            Sinter density                                                                            B     Hc                                     Symbol  (g/cm.sup.3) (g/cm.sup.3)                                                                              (kG)  (Oe)                                   ______________________________________                                        A       7.08         7.45        14.4  0.95                                   B       7.10         7.46        14.6  0.90                                   C       7.10         7.43        14.2  0.97                                   D       7.08         7.45        14.3  0.97                                   E       7.10         7.45        14.3  0.92                                   F       7.14         7.33        13.5  1.11                                   G       7.03         7.30        13.4  1.17                                   ______________________________________                                    

As evident from Table 3, the sintered products obtained according to thepresent invention have a higher sinter density than the sinteredproducts obtained according to the prior art process, and hence, exhibitimproved magnetic properties as soft magnetic material because of highmagnetic flux density (>14 kG) and a lower coercive force (<1 Oe).

INDUSTRIAL APPLICABILITY

As obvious from the foregoing description, the iron-tin composite powderaccording to the present invention has the outstanding benefit of makingpossible the practical manufacturing of tin-containing iron basesintered products or iron-tin sintered products having a high sinterdensity and particularly, improved magnetic properties. Then thecomposite powders according to the present invention are best suited assintering stock materials intended for the production of soft magneticparts useful as cores used in electric apparatus such as motors,mechanical parts requiring high strength and high wear resistance, andthe like.

I claim:
 1. A tin-containing iron base powder comprising iron baseparticles each having tin-rich portions consisting essentially of finelydispersed spots of iron-tin compounds at the surface thereof, whereinthe total tin content is in the range of 1 to 20% by weight of thepowder.